Change magic number for calculation

It's one less than the sizeof(i_data) which is easier to spot than 11.
Binary identical, not tagged.

s/Wimp04

@@ -2050,7 +2050,7 @@ icontext_pushfontstring
         ;Get max index value for menu entry
         TST     R0, #if_indirected
         MOVNE   R3, #bignum
-        MOVEQ   R3, #11
+        MOVEQ   R3, #?i_data - 1
 
         AND     R14,R0,#if_esg
         CMP     R14,#13 :SHL: ib_esg    ; check for esg 13 - if it isn't (could be 12